Presently, many electronics manufacturers are employing an ever-increasing number of surface mount components in their products. Surface mount components, which possess leads or pads ("leads") adapted for bonding to metallized areas on a circuit board, can be made similar than corresponding through-hole components which have leads adapted for insertion in through holes in the board. As a consequence, a circuit board can be populated with a greater number of surface mount components than through-hole components, thus affording increased functionality.
The process of populating a circuit board with surface mount components is initiated by first applying a layer of solder paste onto the metallized areas on the board which to be bonded to the component leads. In practice, the paste, which is usually very viscous, is applied on the circuit board by a technique known as stencil printing. A stencil, having a pattern of openings corresponding to that of the paste pattern to be printed, is placed on the circuit board so that each stencil opening is in registration with a corresponding metallized area on the board. Solder paste is then applied to the stencil and a squeegee blade is thereafter displaced thereacross the stencil to force the paste into the stencil openings for deposit on the circuit board. Once the paste has been deposited (and the stencil is removed), the surface mount components are placed on the circuit board so that the leads of each sit on corresponding solder paste-coated metallized areas on the board. Following component placement, the paste is reflowed, typically by heating the circuit board, to bond the components to the board.
There is now a trend towards reducing the pitch or spacing between the leads to achieve greater lead density for a given component size. Efforts are now under way to reduce the lead pitch of surface mount components from the present-day value of 50 mils (1.27 mm.) to 25 mils (0.625 mm.) and below. Reducing the lead pitch necessitates that the pitch of the stencil openings be correspondingly reduced, which reduces the width of that portion of the stencil (hereinafter referred to as the "web") lying between adjacent stencil openings. As the width of the stencil webs decreases, so does their strength, increasing the likelihood that the webs will buckle, and/or distort as the squeegee passes over them. When the webs buckle, paste seeps underneath them and contaminates the undersurface of the stencil, requiring more frequent stencil cleaning. Distortion of the webs causes the paste pattern deposited on the circuit board to be distorted, which may lead to the formation of bridges between paste deposits on adjacent metallized areas on the circuit board.
Current attempts to solve the problem of printing fine pitch solder paste deposits have incurred difficulties. Making the stencil thicker to strengthen the webs causes the thickness of the solder paste deposits to increase, thereby increasing the amount of paste used, which drives up manufacturing costs. Moreover, increasing the height of the solder paste deposits increases the likelihood that solder bridges may form between adjacent component leads during paste reflow. Using more rigid materials for the stencil, such as stainless steel instead of brass which is conventional, will increase the strength of the webs. However, stainless steel and other such rigid materials are much more difficult to etch so that manufacturing stencils from these materials is often impractical.
Thus, there is a need for a technique for stencil printing a fine pitch pattern of solder paste on a substrate.